The present invention provides a method and apparatus for measuring the placement of material throughout a plastic blow-molded containers and, more specifically, it relates to the use of an infrared camera and a microprocessor to measure the placement of material forming the plastic blow-molded container by comparing the temperature profile images of a bottle selected as a standard and the temperature profile image of a freshly blown bottle.
Although the present invention is described primarily in connection with an extrusion blow molding, the skilled artisan will appreciate that the method of the present invention will also apply to other blow molding processes such as, for example, injection and stretch blow molding processes.
In the packaging industry there is a continuous effort to reduce the overall weight of the package. Weight reduction translates into a reduction of material and shipping costs. One of the hurdles faced by packaging engineers is to ensure that lighter weight packages have the minimum amount of material while meeting specifications for strength and integrity. Achieving this balance often means that there is only a very small margin for error and better automated processes are always being sought.
One of the difficulties in blow molding plastic containers is the problem of maintaining the proper placement of plastic in a parison so that the blow-molded container has uniform wall thickness and/or has the desired amounts of plastic in the desired locations on the container. Ill-placed plastic in a container wall may translate into defects in the containers that, in turn, may affect the structural integrity of the container for its intended use. Moreover, ill-placed material in a container may indicate that there is a serious problem with the process, such as, for example, a clogged screen or foreign objects in the process lines that affect the flow of the plastic.
One way to measure placement of material in a plastic blow-molded container is for an operator to manually remove containers from an output conveyor, cut them open, and measure the vertical wall thickness distribution. If the distribution is not satisfactory, the measurement data can indicate how the process needs to be adjusted. In some cases, there is a critical location where one measurement can indicate how the process is running. For example, if one area of the sidewall is too thick, the additional thickness can indicate that some other area is running too thin.
Wall thickness distribution information must be acquired as quickly as possible. If the manufacturing process is out of specification limits, a large quantity of scrap material can be generated before such determination is made and the appropriate correction implemented. The manual sectioning and measurement of the wall thickness distribution can be tedious, time consuming, and inaccurate. Consequently, the manufacturing process results in a large variation in container wall thickness, rarely approaching the optimum distribution necessary for achieving a desired minimum weight of the final container.
On-line automated processes to measure wall thickness of blow molded plastic containers have been developed to address the foregoing concerns. For example, U.S. Pat. Nos. 6,863,860, 7,374,713, and 7,378,047 to Birckbichler et al. (“the Birckbichler patents”) disclose a method of inspecting wall thickness of blow-molded plastic containers wherein the method involves impinging infrared light thereon and detecting the portion of infrared light that passes through the container and converting the same into corresponding electrical signals that are delivered to a microprocessor. The microprocessor compares the electrical signals with stored information regarding desired wall thickness of the container and emits output thickness information. In such method, the microprocessor receives signals from sensors associated with the blow-molder relating to the position of the molds, the identity of the molds and the identity of the spindles in order that the wall thickness information that is determined by inspection can be associated with specific molds and spindles. This method, however, suffers from at least the following three drawbacks.
First, the method disclosed in the Birckbichler patents requires multiple IR emitters and IR detectors to determine thickness. The emitter sends out the IR wave from one side of the package, while the detector senses the IR wave from the other side of the package. Such configuration is expensive, complex, and difficult to maintain in continuous working operation.
Moreover, since the method disclosed in the Birckbichler patents relies on radiation passing through the container, so the containers must be clear for the method to be effective.
Finally, the method disclosed in the Birckbichler patents takes data points only where the emitter and detectors are aimed and, thus, is incapable of measuring wall thickness continuously (i.e., without interruption) along the side of the container wall.
Accordingly, there is a need in the art for an on-line, automated method of measuring material placement in a blow-molded plastic bottle that does not suffer from the above-identified drawbacks.